Recent surveys of prenatal drug consumption indicate that aspirin is the most frequently consumed drug in pregnancy. Over the past several years, numerous reports have suggested a possible association between prenatal aspirin ingestion and adverse effects in the pregnant woman and her developing fetus. This review summarizes the available experimental animal and human epidemiological data on the possible teratogenicity of aspirin, its effects on fetal lethality, its effects on the duration of pregnancy and parturition, and its ability to alter hemostatic mechanisms in both the mother and newborn. From an analysis of the data, it appears that, although direct conclusive evidence of adverse effects in humans is lacking, a potential hazard does exist and thus "the indiscriminate use of aspirin during pregnancy is contraindicated."